Evolution of frozen magnetic state in co-precipitated Zn^sub d^Co^sub 1-d^Fe^sub 2^O^sub 4^ (0?=?d?=?1) ferrite nanopowders

The evolution of frozen magnetic state of ZnδCo1−δFe2O4 (0 ≤ δ ≤ 1) ferrite nanoparticles was studied by applying vibrating sample magnetometer measurements in temperature range 5–350 K and magnetic fields up to 7 T. It was shown that gradual conversion from the inverse spinel (δ = 0) to the normal one (δ = 1.0) is correlated with a drop of freezing temperature Tf (corresponding to blocking of mean magnetic moment of the system) from 238 K (δ = 0) to 9 K (δ = 1.0) and with a decrease of magnetic anisotropy constant K1 from about 8 · 105 J/m3 to about 3 · 105 J/m3. The percolation threshold predicted for bulk ferrites at 1 − δ ≈ 0.33 was observed as a significant weakness of ferrimagnetic coupling. In this case magnetization curves, determined according to the zero field cooling protocol, reveal two distinct maxima indicating that the system splits into two assemblies with specific ions distribution between A and B sites.